This invention is in the field of elastic wave generation transmission and detection in the earth. More particularly, it is concerned with determining the position in the earth of the drill bit during a drilling operation in a deep borehole. Still more particularly, it concerns determination of the velocity of elastic waves in the earth.
Alternatively, this invention is concerned with initiation of a seismic wave at a selected shallow depth in a water layer overlying the earth's surface for the purpose of determining the geologic structure of the earth to a selected depth below the water-earth interface.
In the prior art, various means have been devised for determining the position of the borehole in the North-South, East-West coordinates, at selected depths during a drilling operation. This has been done by using survey instruments specially designed for introduction into the borehole and sometimes introduced into the drill pipe, which by their internal mechanism make a record of the slope (or angle with the vertical) of the borehole at each of a plurality of selected depths and a measure in relation to the magnetic compass of the azimuth of the slope of the borehole. Knowing the direction of slope and magnitude of slope at each of a plurality of selected known depths and assuming that the borehole is straight inbetween the survey points at which the measurements are made, the profile of the borehole in three dimensions can be plotted.
One serious disadvantage of this particular system is that it requires a stoppage of the drilling process so that the drill pipe can be broken and the survey instruments inserted on a wire line into the drill pipe. Since the cost of operation of the drill rig runs into many thousands of dollars a day, time loss from the drilling operation to make these measurements is extremely expensive. There is understandable reluctance on the part of drillers to make these necessary measurements so that at some future time when this knowledge is needed it will not be available and cannot be easily obtained.